Although there is evidence that dopamine is the hypothalamic prolactin inhibiting factor (PIF), the quantitative relationship between its secretion and that of prolactin has not been established. Using high performance liquid chromatography coupled with an electrochemical detector, dopamine levels in pituitary stalk blood were correlated with prolactin levels in peripheral plasma. Dopamine in plasma was absorbed onto alumina, eluted with 0.5M HCl04 and injected onto a 2 times 1000 mm cation-exchange column. Detection resulted from the oxidation of dopamine at the surface of a carbon paste electrode, with the resulting electrochemical current being linearly related to dopamine from 3-pg -10 ng. With a 50% extraction loss of dopamine, the limit of detection was 0.6 ng/ml when 100 microliters of plasma were assayed. Specificity is provided by alumina absorbing only catechols and the unique retention time of dopamine on the ion exchange column. Pituitary stalk blood was collected for 1 hour from 6 proestrus and 5 diestrus rats using a parapharyngeal surgical route and Nembutal anesthesia. Peripheral plasma for prolactin analysis was obtained just prior to pituitary removal in preparation for stalk blood collection. Dopamine levels in plasma. The ratio of dopamine in stalk vs. peripheral plasma of individual animals ranged from greater than 2 to greater than 38. Dopamine and prolactin levels were not significantly correlated (r equals -0.47; P greater than 0.05). These results demonstrate that dopamine is secreted into pituitary stalk blood but provide no evidence for its dynamic involvement in prolactin secretion.